An integrated circuit chip may contain millions of devices. With such a high packing density, the yield control of the fabrication process is of vital importance. Any defects or damage in the devices on a chip may cause the chip to fail quality inspection. Thus, the yield is greatly reduced with unavoidable defects in densely packed integrated circuits.
One prior art approach to this problem is to form additional spare devices on the chip to replace defective devices. This technique greatly improves the yield of the products and is now widely applied in various applications. One of the major applications is in the fabrication of memory arrays. In the design of memory devices, several redundant rows and columns of memory cells are added to a regular memory array. After the manufacturing processes are finished, a chip probe test is performed to test the function of the devices. The defective devices can be identified and replaced with spare devices by modifying the conductive paths on the chip.
In conventional chip design, a fuse structure is used in rewiring the conductive path to make replacement devices. A fuse bank containing a plurality of fuses are formed on the chip. When a defective memory cell is identified, the fuse addressing the cell is broken and the operating current of the defective device is directed to a defect-free memory cell in the spare rows or columns. Thus, chip defects caused by defective cells can be repaired by identifying and selectively breaking fuses. As an example, if two rows and two columns of redundant cells are used for each memory cell array, the maximum number of defective cells which can be replaced is four.
A widely employed technique in breaking the fuse is to blow the specified fuse by laser energy through a fuse window. However, the use of a laser in blowing the fuses is a time-consuming job with some reliability problems. The identified defective cells must be replaced by applying a laser on each corresponding addressed fuse. The laser beam must be accurately aligned to a correct fuse. The process in blowing the fuses one-by-one is quite time-consuming. Additionally, the energy applied must be well controlled to open the fuse path entirely without forming a residue.
An antifuse technology is proposed as an alternative way to the fuse structure. A programmable antifuse is employed. By programming a fuse to be open or be shorted electrically, the state of each fuse can be changed and the defective cells can be replaced with redundant ones. The same function as the fuse structure can be achieved by detecting the state of each addressed fuse to determine the current path.
The antifuse technology provides a more efficient and reliable way by programming the antifuse electrically versus blowing the fuse physically with a laser. However, the prior art antifuse circuit design requires a complex circuit in programming and detecting the fuse state. An improved antifuse circuit is highly desired for providing the programming and detecting function with device and area saving characteristics.